Can anyone tell me what the difference between the following code1 and code2 is?

code1:

 pheno<-read.table(file="pdata.txt",header=T,row.names=1)
    head(d)
         subgroups batch
    sample1   1     1
    sample2   1     1
    sample3   1     1
    sample4   4     1
    sample5   1     1
    sample6   4     1
    batch<-pheno$batch
    subgroups<-pheno$subgroups
modcombat1<-model.matrix(~1, data=pheno)
combat_mydata<-ComBat(dat=mydata, batch=batch, mod=modcombat1, par.prior=TRUE, prior.plots=FALSE)

code2:

 pheno<-read.table(file="pdata.txt",header=T,row.names=1)
        head(d)
             subgroups batch
        sample1   1     1
        sample2   1     1
        sample3   1     1
        sample4   4     1
        sample5   1     1
        sample6   4     1
        batch<-pheno$batch
        subgroups<-pheno$subgroups
    modcombat2<-model.matrix(~subgroups, data=pheno)
    combat_mydata<-ComBat(dat=mydata, batch=batch, mod=modcombat2, par.prior=TRUE, prior.plots=FALSE)

To put it simply,what is the difference in mod with or without the covariate level? What is the purpose of including covariates in the mode? And which one should i choose for only batch effects removal but biological variation from the subgroups retained? Many thanks.